Non-linear resistor and method of manufacturing the same

ABSTRACT

A non-linear resistor having an operational field strength which optionally is formed as a VDR- or as an NTC-resistor having a ceramic sintered body on the basis of a polycrystalline alkaline earth metal titanate doped with a small quantity of a metal oxide so as to produce an N-type conductivity, in which the sintered body comprises at its grain boundaries insulating layers formed by re-oxidation of the sintered body and consists of an alkaline earth metal titanate having a Perowskite structure of the general formula 
     
         (A.sub.1-x Ln.sub.x)TiO.sub.3.yTiO.sub.2 or A(Ti.sub.1-x 
    
      Me x )O 3 .yTiO 2 , 
     wherein: A=alkaline earth metal; Ln=rare earth metal; Me=metal having a valency of 5 or more; 0.0005&lt;×&lt;solubility limit in the Perowskite phase; y×0.001 to 0.02. The sintered body becomes adjustable in its non-linear resistance variation by selection of the re-oxidation temperature and of the re-oxidation duration in such manner that an initially present NTC-characteristic gradually is observable only at ever increasing temperatures and changes into a VDR-characteristic in the range of the operating temperature of the resistor.

The invention relates to a non-linear resistor having a ceramic sinteredbody on the basis of a polycrystalline alkaline earth metal titanatedoped with a metal oxide to produce an N-type conductivity, the bodyhaving electrodes provided on oppositely located surfaces. The inventionfurthermore relates to a method of manufacturing such a resistor.

Non-linear resistors are to be understood to mean in this case resistorshaving an NTC-characteristic (resistance value decreases independentlyof the applied voltage with increase in temperature) and resistorshaving a VDR-characteristic (resistance value depends only on theapplied voltage).

From U.S. patent application Ser. No. 263,321 (=published EP-PA No.40,881) a voltage-dependent resistor is known which is based on N-dopedstrontium titanate to which prior to sintering a small quantity of alead germanate phase was added which leads to the formation ofinsulating grain boundary layers in the polycrystalline grain texture ofthe sintered body. Due to its comparatively high operational fieldstrength--a current density, for example, of approximately 3 mA/cm² isobtained only with fields of approximately 6 kV/cm--this known resistorhas only a limited field of application; for example, it is not suitablefor modern semiconductor switching circuits operating at low voltages.

It is the object of the invention to provide a non-linear resistor asmentioned above and a method of manufacturing same in such a manner thatnot only a non-linear resistor having a low operational field strengthis obtained, but that said resistor is formed optionally as a VDR or asan NTC-resistor.

According to the invention this object is achieved when the sinteredbody comprises at its grain boundaries insulating layers formed byre-oxidation of the sintered body and consists of an alkaline earthmetal titanate containing excess TiO₂ and having a Perowskite structureand a composition defined by one of the general formulae

    (A.sub.1-x Ln.sub.x)TiO.sub.3.yTiO.sub.2 or A(Ti.sub.1-x Me.sub.x)O.sub.3.yTiO.sub.2

in which:

A=alkaline earth metal

Ln=rare earth metal, including yttrium

Me=metal having a valency of 5 or more

0.0005<x< solubility limit in the Perowskite phase

y=0.001 to 0.02.

A method of manufacturing the sintered body of the above-mentioned typeis carried out according to the invention so that the ceramic body isfirst sintered in a reducing atmosphere and that said sintered body isthen re-oxidized in an oxidizing atmosphere, preferably in air, in whichthe sintered body, by choice of the reoxidation temperature and thereoxidation duration, is adjustable in its non-linear resistancevariation such that an initially present NTC-characteristic as afunction of the oxidation state gradually can be observed only atincreasing temperatures and, in the range of the operating temperatureof the resistor, changes into a VDR-characteristic.

As a result of the sintering in a reducing atmosphere the sintered bodyis made continuously semiconductive and subsequently grain boundarylayers of the semiconductor grains of the polycrystalline grainstructure of the sintered body are converted by the formation ofhigh-ohmic oxide layers by re-oxidation. In accordance with the value ofthe reoxidation temperature and the reoxidation duration, sinteredbodies can thus be manufactured at will in which the NTC-characteristicor the VDR-characteristic predominates.

According to advantageous modified embodiments of the invention,strontium is chosen as an alkaline earth metal and La₂ O₃, Nb₂ O₅ or WO₃are chosen as doping metal oxides. The incorporation of the doping metaloxide in the Perowskite lattice of the SrTiO₃ occurs by reaction alreadyduring the pre-sintering in the manufacture of the sintered body. Inaddition to the said dopants, other metal oxides are also feasible, forexample, Y₂ O₃, Sm₂ O₃, Ta₂ O₅, As₂ O₅, Sb₂ O₅, MoO₃ or U₃ O₈.

In accordance with the ion radius the doping ions are incorporatedeither in Sr-sites or in Ti-sites in the Perowskite lattice of theSrTiO₃. It has been demonstrated by means of X-ray structure analysisthat the large La³⁺ -ion (r_(La) 3+=0.122 nm) is incorporated inSr-sites (r_(Sr) 2+=0.127 nm). By analogous studies on PbTiO₃ it couldbe demonstrated that the smaller Nb⁵⁺ -ion (r_(Nb) 5+=0.069 nm) isincorporated in Ti-sites (r_(Ti) 4+=0.064 nm). On the basis of the ionradius of the W⁶⁺ -ion (r_(W) 6+=0.062 nm) it can accordingly beconcluded that it is also incorporated in Ti-sites.

After the presintering the doped alkaline earth metal titanate isbrought into a sinterable finely pulverized state by grinding in a ballmill and is formed into usually a disc-shaped body by compression. Onlywhen sintering is carried out in a reducing atmosphere do the donorcharges contribute directly to the conductivity. This condition isreferred to as electron compensation. The chemical characterization ofsuch electron-compensated, semiconductor Perowskite phases with n-dopingfollows for the dopings according to the invention: ##EQU1##

The electron-compensated materials have a resistivity in the order ofmagnitude of 1 Ωcm.

When on the contrary the samples are sintered in an oxidizingatmosphere, the compensation of the donor charges occurs via cationvacancies, mainly vacancies in Sr-sites. Such vacancy-compensatedmaterials are highly insulating since the cation vacancies operate asvery strong electron acceptors. The chemical characterization ofvacancy-compensated materials are as follows for the dopings accordingto the invention:

Sr_(1-3x/2) La_(x).sup.· □"_(x/2) TiO

Sr_(1-x/2) Δ"_(x/2) (Ti_(1-x) Nb_(x).sup.·)O₃

Sr_(1-x) Δ_(x) "(Ti_(1-x) W_(x).sup...)O₃

'=symbol for acceptor electron

□=symbol for lattice vacancies

These vacancy-compensated materials have a resistivity in the order ofmagnitude of 10¹³ Ω.cm.

The invention is based on the recognition of the fact that theelectron-compensated semiconductor ceramic can be converted into thehighly insulating vacancy-compensated form by re-oxidation. A variety oftransition states can be produced between the purelyelectron-compensated form which corresponds to a Perowskite ceramichaving NTC-properties, and the purely vacancy-compensated form whichcorresponds to a Perowskite ceramic having VDR-properties. Reactionkinetic experiments on electron-compensated semiconductive Ba_(1-x)La_(x) .sup.. TiO₃ (z≈0.005 to 0.02) have demonstrated that theoxidation always begins at the grain boundaries and a semiconductorceramic material having highly insulating grain boundary layers isformed. Analogous processes occur in the oxidation of semiconductorN-doped SrTiO₃.

A particular advantage which is obtained with the invention is the lowoperational field strength in resistors having VDR-characteristic inaddition to the adjustability of the characteristic of the resistorscomprising a ceramic sintered body according to the present Application.As compared with the known resistor according to U.S. patent applicationSer. No. 263,321, now U.S. Pat. No. 4,417,227, the resistors havingVDR-characteristic according to the present Application aredistinguished by an operational field strangth which is lower by afactor of ≦20.

As a result of this, varistors with sintered bodies according to thepresent invention become particularly applicable for modernsemiconductor switching circuits operating at low voltages. Thevaristors having sintered bodies with La-doping, Nb-doping or W-dopingall show the said low operational field strength. For this it isimportant for the sintered body to have a low TiO₂ -excess and tocomprise insulating layers formed by re-oxidation. These insulatinglayers may show a gradient in the resistivity from the edge zones of thesintered body over the thickness of the sintered body.

Grain growth of the titanate starting material during sintering dependsupon the presence of an excess of TiO₂ the dopant concentration and thesintering conditions particularly the sintering temperature. The grainsize of the polycrystalline structure has a decisive influence on theoperational field strength of the non-linear resistor. The smaller thegrain size, the higher is in general, the operational field strength ofthe resistor.

It should be noted, however, that when the operational field strength istoo low the current index β assumes increasingly unfavourable values.The current index β appears from the formula U=C . I.sup.β, whereI=current through the resistor in amperes; U=the voltage drop across theresistor in volts; C=a geometry-dependent constant, it indicates thevoltage at I=1 A (in practical cases it may assume values between 15 anda few thousand); β=current index, coefficient on non-linearity orcontrol factor which is material-dependent and is a measure of thesteepness of the current-voltage characteristic. Preferably the β valueshould be as small as possible, because at a small value of β largecurrent variations lead only to small voltage variations across thenon-linear resistor.

Embodiments of the invention and their operation will now be describedin greater detail with reference to the drawings, in which

FIG. 1 shows a current-voltage characteristic of a non-linear resistoraccording to the invention,

FIGS. 2a to 2c show the dependence of the operational voltage with acurrent of 1 mA and of the current index β on the re-oxidationtemperature for different non-linear resistors according to theinvention.

FIG. 3 is a diagrammatic representation of the current and temperaturedependence of the electric resistance of a non-linear resistor inaccordance with the invention, and

FIG. 4 shows the current and temperature dependence of the electricalresistance of a non-linear resistor in accordance with the invention atdifferent re-oxidation temperatures.

The manufacture of non-linear resistors according to the presentinvention will first be described hereinafter.

1. Manufacture of the ceramic sintered bodies

SrCO₃ and TiO₂ were used as starting materials for the ceramic sinteredbody and La₂ O₃, Nb₂ O₅ or WO₃ were used as doping metal oxides. In thepreparation of the ceramic mass according to the compositions (Sr_(1-x)La_(x))TiO₃.yTiO₂, Sr(Ti_(1-x) Nb_(x))O₃.yTiO₂ or Sr(Ti_(1-x)W_(x))O₃.yTiO₂ with 0.0005<x<solubility limit in the Perowskite phaseand y=0.001 to 0.02, the TiO₂ -excess with 0.001 to 0.02 has hence beenchosen that there is always a small excess of Ti⁴⁺ -ions. As a result ofthis, a liquid sintering phase with the SrTiO₃ is formed at a sinteringtemperature above 1400° C.--it is assumed that it deals with theeutectic SrTiO₃ --TiO₂ occurring at ≈1440° C., which may also occur atlower temperatures by the addition of dopants. A liquid sintering phaseof this type favours the desired coarse granular grain growth.

For the manufacture of the ceramic bodies the following quantities wereweighed-in.

For Sr(Ti₀.99 Nb₀.01)O₃.0.01TiO₂ :

59.056 g of SrCO₃

31.96 g of TiO₂ and

0.5316 g of Nb₂ O₅ ;

for (Sr₀.99 La₀.01)TiO₃.0.01TiO₂ :

58.465 g of SrCO₃

32.28 g of TiO₂

0.6517 g of La₂ O₃ ;

for Sr(Ti₀.996 W₀.004)O₃.0.01TiO₂ :

59.056 g of SrCO₃

32.152 g of TiO₂

0.371 g of WO₃.

They were mixed wet in a ball mill, for example, of agate. Pre-sinteringat 1150° C. for 15 hours is then carried out. The pre-sintered powder isagain ground while wet (1 hour in a ball mill, for example, of agate).The ground material is then dried and the powders thus obtained are thengranulated by means of a suitable binder, for example, a 10% aqueouspolyvinyl alcohol solution. The granulate is then compressed, forexample, into discs having a diameter of ≈6 mm and a thickness ≈0.50 mmwith a green density (density after compression) of approximately 55 to60% of the theoretical density. Sintering of the pressed product is thencarried out at a temperature of 1460° C. for 4 hours in a reducingatmosphere. The atmosphere may consist, for example, of watervapour-saturated mixed gas consisting of 90% by volume of N₂ and 10% byvolume of H₂. Since the oxygen partial pressure of the mixed gas isdetermined by the ratio of the two partial pressures P_(H).sbsb.2/P_(H).sbsb.2_(O), the mixed gas should be saturated with H₂ O at ≈25°C. so as to create a standard reducing atmosphere. During the sinteringit should be ensured that coarse granular structures occur preferably atsintering temperatures above 1440° C.

The reducing sintering is to be carried out in a closed furnace, forexample, a tubular furnace is suitable. Excess reducing gas shouldpreferably flow away through a bubble counter so as to create a stablesintering atmosphere.

Sintered bodies manufactured in this manner are semiconductive and showno open porosity anymore.

By re-oxidation of these sintered bodies in an oxidizing atmosphere, forexample in air, electrically highly insulating oxide layers as grainboundary layers are produced in the semiconductor grain structures ofthe sintered bodies. Experiments which led to the present invention werecarried out in different conditions:

(a) at a fixed re-oxidation duration of 120 minutes at differenttemperatures of 900° C., 1000° C., 1100° C., 1200° C., or 1300° C.

(b) at a fixed temperature of 1100° C. and different re-oxidationdurations of 5 minutes, 15 minutes, 30 minutes, 60 minutes, 120 minutesor 240 minutes.

2. Manufacture of non-linear resistors

Electrodes of suitable metals, preferably of gold, were provided, forexample by vapour deposition, on sintered bodies prepared as describedabove so as to form a resistor. For better adhesion of the electrodemetal it is recommendable first to provide on the ceramic body asuitable adherent layer as an intermediate layer between ceramic andelectrode metal; for example, a Cr-Ni-layer is suitable.

Notes for special compositions:

(Sr_(1-x) La_(x))TiO₃.yTiO₂ (0.0005<x<solubility limit of the La in thePerowskite phase; y=0.001 to 0.02): when x<0.0005, the bodies to besintered oxidize too rapidly, and the reproducibility of the results isno longer ensured.

The upper limit of x appears from the solubility limit of the La in thePerowskite phase. Optimum results were achieved with sintered bodieshaving a grain structure with grains of a diameter of 80 to 120 μm withx=0.01 and y=0.01 at a sintering temperature of 1460° C. in a reducingatmosphere.

    Sr(Ti.sub.1-x Nb.sub.x)O.sub.3.yTiO.sub.2 (0.005<x<solubility

limit of the Nb in the Perowskite phase; y=0.001 to 0.02): the same asdescribed above for the La-dopings also applies to the lower limit of x;from x≈0.03 and more, homogeneous microstructures were no longerobtained in a reproducible manner. Optimum results were achieved withsintered bodies having a grain structure with grains of a diameter of 60to 80 μm with x=0.01 and y=0.01 at a sintering temperature of 1460° C.in a reducing atmosphere.

    Sr(Ti.sub.1-x W.sub.x)O.sub.3.yTiO.sub.2 (0.0005<x<solubility

limit of the W in the Perowskite phase; y=0.001 to 0.02): the same asdescribed above for the La-dopings also applies to the lower limit of x;from x≈0.01 predominantly fine-granular microstructures were observed,from x≈0.06 and more a separation of foreign phases occurs increasinglyin the microstructure which consists of SrWO₄ and TiO₂. Optimum resultswere achieved with sintered bodies having a grain structure with grainsof a diameter of 60 to 80 μm, with x=0.004 and y=0.01 at a sinteringtemperature of 1460° C. in a reducing atmosphere.

3. Results

FIG. 1 shows the current-voltage characteristic of a resistor havingVDR-characteristic with a sintered body of the composition(Sr_(O).sbsb.2₉₉ La₀.01)TiO₃.0.01TiO₂. The current density in mA/cm² isplotted against the applied electrical field strength in kV/cm. In orderto adjust the VDR-characteristic, the sintered body was re-oxidized inan oxidizing atmosphere at 1300° C. for 2 hours after having beensintered in a reducing atmosphere, as already described. The sinteredbody had a cross-sectional area of approximately 0.19 cm² and athickness of approximately 400 μm.

FIGS. 2a to 2c show the operational voltages at a current of 1 mA andthe current indices β for resistors having sintered bodies of thefollowing compositions dependent on the re-oxidation temperature:

FIG. 2a: Sr(Ti₀.99 Nb₀.01)O₃.0.01TiO₂

FIG. 2b: (Sr₀.99 La₀.01)TiO₃.0.01TiO₂

FIG. 2c: Sr(Ti₀.9996 W₀.004)O₃.0.01TiO₂.

The numbers beside the points in the individual curves indicate thetemperatures at which re-oxidation was carried out in an oxidizingatmosphere for 2 hours. A β=1 means a purely ohmic resistance withNTC-properties (in FIGS. 2a to 2c the operational voltage is plottedlogarithmically so that values for β=1 are not shown within the range ofU-values given). Of all three material groups this value is reachedprior to the re-oxidation and at low re-oxidation temperatures,respectively (see Tables 1 and 2). With the geometry chosen theresistance of the sintered body prior to the re-oxidation typically liesat values of a few ohms. This results in a resistivity of approximately10 Ω.cm. FIGS. 2a to 2c furthermore show that when the re-oxidationtemperature is increased the β value decreases very markedly, so atypical VDR-behaviour begins. In particular the sintered bodies dopedwith La show that already at low operational voltages of a few volts βvalues of 0.16 were achieved. By further increasing the re-oxidationtemperature, the operational voltage can be varied in a wide range withβ approximately constant.

FIG. 3 shows diagrammatically general pattern of the current andtemperature dependence of the electric resistance for non-linearresistors according to the invention. In (R) (the logarithm of theelectric resistance R) is plotted against the reciprocal temperature1/T. The range I characterizes a pure NTC-behaviour, the range IIcharacterizes a VDR-behaviour.

A range of constant increase in such a diagram results in a relation ofR and T according to R=A. e^(B/T)

(R=resistance, measured in Ω;

A=a constant having the dimension Ω essentially dependent on the outershape of the resistor;

e=base of the natural logarithm;

B=a constant having the dimension K dependent on the outer shape and onthe NTC-material;

T=absolute temperature of the resistor in K). In wide temperature rangessuch a linear relation is given. In principle, however, two ranges areto be distinguished. A range I in which a high value of B, that is astrong temperature dependence, is to be observed with a simultaneouslyabsent dependency on the applied current (NTC-behaviour) and a range IIin which InR exhibits a flat characteristic, that it has a smalltemperature dependence. In this case, however, a considerable influenceof the current on the resistance is to be noted, so this is the range inwhich VDR-properties predominate. Both ranges merge into each other witheach time different current values and temperatures.

FIG. 4 shows graphically the current and temperature dependence of theelectric resistance of a non-linear resistor of the composition (Sr₀.99La₀.01)TiO₃.0.01 TiO₂. The logarithm InR of the electric resistance (R)is plotted against the reciprocal absolute temperature. The sinteredbodies were re-oxidized in air at temperatures of 1100° C. and 1200° C.,respectively, for 2 hours after having been sintered in a reducingatmosphere as described above.

The influence of the higher re-oxidation temperature is experienced herein a shift of the set of curves to higher resistance values.

The following Table 1 gives the numerical values of the non-linearityfactor β and the operational voltage U_(1mA) for individual dopants andre-oxidation temperatures.

                                      TABLE 1                                     __________________________________________________________________________    Re-oxidation                                                                           Sr(Ti.sub.0.99 Nb.sub.0.01)O.sub.3.0.01 TiO.sub.2                                           (Sr.sub.0.99 La.sub.0.01)TiO.sub.3.0.01                                                     Sr(Ti.sub.0.996 W.sub.0.004)O.sub.3.0                                         .01 TiO.sub.2                            temperature (°C.);                                                                     Operational   Operational                                                                          current                                                                              operational                       (Re-oxidation                                                                          Current index                                                                        voltage                                                                              Current index                                                                        voltage                                                                              index  voltage                           duration 2h)                                                                           β U.sub.1mA (V)                                                                        β U.sub.1mA (V)                                                                        β U.sub.1mA (V)                     __________________________________________________________________________     900     0.75   0.09   1      0.002  0.44   0.5                               1000     0.6    0.21   0.94   0.017  0.19   5.1                               1100     0.57   0.47   0.34   0.5    0.19   7.3                               1200     0.24   3.2    0.16   2.2    0.15   12.5                              1300     0.13   20.6   0.17   6.6    0.13   16.8                              __________________________________________________________________________     Thickness of the sintered bodies uniformly ≈ 400 μm                Diameter of the sintered bodies uniformly ≈ 5 mm.                

Table 2 shows the influence of the re-oxidation duration at a constanttemperature of 1100° C. In this case also it is confirmed that anincreasing strength of the re-oxidation given by a higher temperature orlonger re-oxidation duration, influences the values for the currentindex β and the operational voltage U_(1mA) (see also FIGS. 2a to 2c).

                                      TABLE 2                                     __________________________________________________________________________    Re-oxidation                                                                             Sr(Ti.sub.0.99 Nb.sub.0.01)O.sub.3.0.01 TiO.sub.2                                           (Sr.sub.0.99 La.sub.0.01)TiO.sub.3.0.01                                       TiO.sub.2     Sr(Ti.sub.0.996 W.sub.0.004)O.sub.3                                           .0.01 TiO.sub.2                        (min)             Operational   Operational                                                                          current                                                                              operational                     (re-oxidation                                                                            Current index                                                                        voltage                                                                              current index                                                                        voltage                                                                              index  voltage                         temperature 1100° C.)                                                             β U.sub.1mA (V)                                                                        β U.sub.1mA (V)                                                                        β U.sub.1mA (V)                   __________________________________________________________________________     5         0.68   0.13   1      0.003  0.27   1.4                              15        0.59   0.22   1      0.005  0.21   2.5                              30        0.64   0.23   1      0.010  0.22   3.0                              60        0.57   0.36   0.75   0.07   0.15   5.7                             240        0.62   1.6    0.2    1.1    0.15   7.2                             __________________________________________________________________________     Thickness of the sintered bodies uniformly ≈ 400 μm                Diameter of the sintered bodies uniformly ≈ 5 mm.                

FIG. 5 shows a cross-section through a nonlinear resistor with a ceramicsintered body (1) according to the invention. The body is on both sidesprovided with electrode-layers (2), and metal electrode caps 3, on whichconnection leads (4) have been secured.

What is claimed is:
 1. A non-linear resistor having a ceramic sinteredbody consisting of a polycrystalline alkaline metal titanate doped toN-type conductance with a metal oxide and which sintered body isprovided with electrodes on opposite located surfaces thereofcharacterized in that said body has a Perowskite structure, consists ofan alkaline earth metal titanate containing excess TiO₂, has a formulaselected from the group consisting of (Sr_(1-x) Ln_(x)) TiO₃.yTiO₂ andSr(Ti_(1-x) Me_(x))O₃.yTiO₂ wherein Ln is a rare earth metal, Me is ametal having a valence of at least 5, 0.0005<x<solubility limit of theLn in the Perowskite phase and y=0.001 to 0.02 and which sintered bodyis provided on its grain boundaries with insulating layers produced byre-oxidation.
 2. A non-linear resistor as claimed in claim 1,characterized in that lanthanum is the rare earth metal.
 3. A non-linearresistor as claimed in claim 1, characterized in that niobium is themetal of a valance of at least
 5. 4. A non-linear resistor as claimed inclaim 1, characterized in that tungsten is the metal of a valance of atleast 5.